There are currently two special types of quests: Interactive and Radiant. All Interactive quests are Radiant but not all Radiant quests are Interactive.
Interactive:
Interactive quests are quests that are... well... interactive. Admins or mods will log on into the General NPC account, take on whatever form is required (i.e; A Seraph zealot in the Interactive quest "Seraph Witness"), and will role play with you.
Radiant:
Radiant quests are simply quests that can be done as many times as you want.